The present disclosure is directed to a milling machine for milling long sections from a cased well. It particularly finds application were a well has been operated for a substantial period of time at a specified depth and is ultimately depleted so that reentry is necessary. It is especially helpful where reentry and recompletion involves horizontal drilling, namely, deviation of the well from the original vertical course of the well. Briefly, consider as an example a well that is extended into a particular formation at a depth of 8,000 feet. Assume that the formation production slumps, and becomes unprofitable. Assume further that the formation is known to be a productive formation. In this instance, it is desirable to extend the well to another portion of that same formation. This can be accomplished by well reentry above the formation, drilling on a curvature to reenter the formation with a well segment which is approximately horizontal. To do this, the well must be reentered a few hundred feet above the formation, and that typically requires cutting away a portion of the casing that was placed in the well to the depth of 8,000 feet or greater. In that instance, reentry must begin by cutting away the casing so that drilling through the sidewall can be accomplished after the subsequent recompletion process begins with removal of a window of the casing. Sometimes, two or three hundred feet of the casing must be removed. Practically speaking, the only process for removal is cutting it away in the fashion of milling. Milling machines are known. The present apparatus, however, is an improved miller which enables the milling of several hundred feet, perhaps two or three hundred feet of casing, and this can be done by operating in a continuous fashion. It enables longer knives to be used, permitting greater wear on the knives, and thereby reducing trips into the well to service the cutter mechanism. It is operated in response to pressure in the drill string. Moreover, it provides a set of knives which extend radially outwardly, thereby enabling the tool to enter through a relatively narrow previously cased well.
On the latter point, consider the typical dimensions. When the well is first drilled, it typically will be drilled with a drill bit forming a hole of perhaps seven and one half inches or larger diameter. Subsequently, a casing string will be placed in the well and cement will be placed in the annular space on the exterior of the casing. This will leave an internal passage through the casing which is much smaller, perhaps in the range of about five inches diameter. When a reentry program is initiated, the milling device must be lowered on a string of drill pipe where all this equipment fits within the cased well. In other words, clearance is tight, and the room necessary for easy rotation during drilling procedures at reentry is very snug.
To this end, the present apparatus provides a milling device which has retracted knives which are not extended until they are at the depth in the well where extension is necessary. It is extremely helpful to have this improved clearance tool so that reentry at great depths can be accomplished. Reentry is thus initiated by milling a long window in the casing. In this particular instance, the present invention enables reentry and cutting of a long window, even as long as three hundred feet. Typically, this can be accomplished with a single trip into the well after cut out is first made thereabove.
The present apparatus is thus a milling device which includes an elongate outer body. There is an internal mandrel within the body and it is mounted for movement between upper and lower positions. The mandrel is positioned adjacent to and beneath a set of knife blades. The knife blades are mounted so that they can extend radially outwardly. They have a first position where the knife blades are retracted; alternately, they are extended by upward movement of the mandrel. The mandrel is forced into the operative position, causing knife blade extension, by application of increased pressure in the drill string. The tool incorporates a drill string pressure fluid receiving chamber, and the chamber, on expansion, forces the mandrel to move upwardly. The mandrel in constructed with an external surface including an enlarged shoulder which slides under or beneath the set of knife blades, forcing them radially outwardly and moving the knife blades into a cutting position.
The present apparatus further is able to thereafter begin milling, and to mill the casing by continuous rotation of the drill string. This can continue as long as required so that the casing is removed for a designated length. It is not uncommon to require a milled window of three hundred feet in length. If the number of trips can be reduced by the use of the present invention, a very desirable result is accomplished, namely, the milling process can be accomplished much more rapidly.
The present disclosure sets forth an apparatus which threads to the lower end of a drill string to fit within a cased hole. It incorporates a top sub for connection with the drill string. A drain sub is threaded to that and supports an internal drain sleeve. A circulating port connects through the two of these to an internal axial passage. A drain plug fits in the axial passage and has a narrow passage therethrough, and is attached by means of a shear pin. The drain sub is activated only if the tool cannot retract the knives, after milling, with ordinary procedures. When the drain sub plug shears, it opens ports for circulation, and also pushes the piston into the relaxed position. This can be sheared on dropping a sphere into the drill string for closure purposes. The drain sub connects with a surrounding outer body extending therebelow. That encloses a telescoped mandrel. The mandrel connects with the axial passage, has a protruding stinger, and defines an axial passage the full length of the mandrel to a chamber at the lower end of the mandrel. This chamber serves as a fluid expansion chamber. When fluid is introduced into this chamber under pressure, it forces the mandrel upwardly. This chamber is selectively connected to the exterior by means of matching ports which align on upward movement of the mandrel so that fluid can flow into the chamber and into the annular space on the exterior of the tool for flowing upwardly to wash cuttings away from the knives. There is an annular space around the mandrel located within the outer body to receive a set of knives, and the knives extend radially outwardly when the mandrel moves upwardly. The knives are supported for movement outwardly. At the front end of the knives, they are guided by a dovetail arrangement to direct the knives outwardly. The mandrel is provided with changes in diameter defining a set of slots or steps which guide the back ends of the respective knife blades. This extends the knife blades outwardly and locks them so that they are held radially outwardly for cutting exteding through ports or slots formed in the tubular outer body. This operates based on a shoulder means forcing the knives radially outwardly in response to upward movement of the mandrel.